Hi had cords too (ugly, like spiderwebs that seemed to suddenly appear in the arm pits after 7 or 8 days post surgery ). They were painful and I felt so ugly. I then had a good regime of exercises prescibed by the hospital outpatients physio, one who specialised in lymphoedema management. . Initally, she asked my husband to help with the stretches, because I could not quite manouvre effectively.. So he copied her. He put dimethicone cream over the stiff corded bits and pressed and massaged firmly with his thumbs. The cords settled completely after several months, but I could still feel where they were until recently.
I found that stretching regularly several times every day (" twinkle twinkle little stars", "angel wings") and swimming ( as soon as scars healed) a few times per week helped me, almost miracoulously, turn the corner and get my full movement back. (I really thougth I miracle had hit me after a 3 sessions of laps in the pool!) I even felt them 'pop' a few times.... Wierd, but nothing to worry about.
I especially like to stretch my arms up the wall when i am in the shower. I stretch my arms up in the air before I get out of bed. I like to move every joint before I get up - shoulders to fingertips. Driving and sitting around makes the stiffness worse. In the car, I stretch my arm over the back of the other seat. I do the same ot the movies... and avoid having someone sit on my 'surgery side', if possible. I'm still prone to stiffening up, 18 months down the track, so i am still stretching, still swiming and I feel so much more in control of my body.
I have heard that many Tai chi moves are also really good for all joints. That migth suit you too.
I'm not on Tamoxifen, but i am on Arimidex... so now I annoyed with stiffness everywhere else, not just my right arm. So i think I'll be stretching for years!
I hope this helps, Cheers, Fiona